With the ever increasing volumes of image data and with ever increasing demand for higher-resolution images, there is a continuing need for reducing further the size of compressed image data. JPEG2000 compression/decompression is a significant improvement over standard JPEG compression/decompression. It can yield equivalent image quality but with much smaller compressed file size. JPEG2000 compression/decompression is capable of supporting both lossless and lossy compression. Lossless compression allows the exact original data to be reconstructed from the compressed data but at the expense of lower efficiency. Lossy compression is more efficient, producing smaller compressed image files but cannot guarantee to reproduce exactly the same image data.
A reduction in the size of image data may be achieved where the image is split into a series of areas, referred to as “tiles”, and where only those tiles in which the image has changed are sent. According to this scheme, when only a small, localised part of the whole image of, say, 16 tiles has changed, it may be that only three of the 16 tiles need to be provided. These three tiles can then be overlaid on the previous image in order to generate the full image. However, this technique will only work well with JPEG2000 if lossless compression is used. If lossy compression is used, tile boundary artefacts appear at the boundaries between the changed tiles and the tiles imported from the previous image. These artefacts display a noticeable tile boundary and can result in a reduction in perceived image quality. The tile boundaries become more noticeable the higher the rate of compression.
It is therefore desirable to provide a method and system for processing image data so as to reduce the size of compressed image data without introducing tile boundary artefacts at high compression rates.